The present invention relates to a picture display apparatus, in particular to a projection type picture display apparatus using reflective type liquid crystal display means.
FIG. 3 is an explanatory view indicating a conventional system for use as a projection type picture display apparatus. As shown in FIG. 3, a white light from a light source 1 is incident on a YDN (Yellow Reflecting Dichroic Mirror) 2a to be divided into a blue light which is a rectilinear light rectilinearly propagated through the YDM 2a, and a two-color light (including a green light and a red light) reflected from the YDN 2a.
Referring again to FIG. 3, the blue light rectilinearly propagated through the YDM 2a is further reflected by a reflecting mirror 3a and then incident on a light transmissible liquid crystal panel 4a (for use in processing a blue light). On the other hand, the two-color light (including a green light and a red light) is incident on a GDM (Green Reflecting Dichroic Mirror) 2b to be divided into a green light and a red light, respectively. Here, the green light is a reflected light from the GDM 2b, whilst the red light is a rectilinear light rectilinearly propagated through the GDM 2b. The green light reflected from the GDM 2b is incident on a light transmissible liquid crystal panel 4b (for use in processing a green light). On the other hand, the red light rectilinearly propagated through the GDM 2b is reflected by a reflecting mirror 3b and further by a reflecting mirror 3c so as to be incident on a light transmissible liquid crystal panel 4c (for use in processing a red light).
The light transmissible liquid crystal panel 4a (for use in processing a blue light), the light transmissible liquid crystal panel 4b (for use in processing a green light), the light transmissible liquid crystal panel 4c (for use in processing a red light), each comprises a pair of transparent electrode substrates forming an internal space therebetween which is filled with a liquid crystal so as to form a plurality of liquid crystal cells. Further, each of the light transmissible liquid crystal panels 4a, 4b, 4c includes on both sides thereof a pair of polarizing plates which are provided to cause a blue, green or red light to be incident onto a light synthesizing cross-prism 5.
The light synthesizing cross-prism 5 has a pair of dichroic mirrors 5a, 5b each consisting of a deposited film, which are arranged in a mutually crossed manner so that they are able to reflect or transmit various lights (blue, green and red lights).
In this way, a red light is allowed to pass through the dichroic mirror 5b and then reflected by the dichroic mirror 5a. A blue light is reflected by the dichroic mirror 5b and then is allowed to pass through the dichroic mirror 5a. Further, a green light is allowed to first pass through the dichroic mirror 5b and then through the dichroic mirror 5a. Therefore, various lights (blue, green and red lights) are synthesized in the light synthesizing cross-prism 5 to form a picture which is then enlarged through a projection lens 6 so as to be projected on a screen 7.
However, with the above-described conventional system shown in FIG. 3, since a high luminance lamp such as a xenon lamp or a halogen lamp is used as a light source, such a light source produces a considerable amount of heat. Further, since the polarizing plates allow the passing of some polarized lights but absorb other polarized lights, a considerable part of the incident light will be heated, hence causing a rise in the temperature of the optical system.
To cope with the temperature rising in the above optical system, it is possible to provide a cooling fan under an optical chassis fixedly mounting the polarizing plates, liquid crystal panels, and dichroic mirrors, so as to cool these optical elements by way of several holes formed through the optical chassis. This, however, will undesirably cause an increase in the total size of the optical system. If a clearance between the cooling fan and the apparatus frame structure is designed to be very small in order to inhibit an increase in the total size, there will be too large a resistance when a cooling air flows therethrough, resulting in a low cooling efficiency. Moreover, the use of a cooling fan will cause a noise which however should be avoided with a picture display apparatus.